The present invention relates to a linear gauging head, including support and protection means with a casing, defining a longitudinal geometrical axis, an elongate element, axially movable with respect to the support and protection means, thrust means arranged between the support and protection means and the elongate element to apply a substantially axial thrust to the elongate element, guide means, for guiding axial displacements of the elongate element with respect to the casing, including at least an axial bearing arranged within the casing and including a plurality of rolling elements cooperating with the elongate element, and a position transducer for detecting displacements of the elongate element with respect to the support and protection means.
Axial movement linear gauges, or gauging heads, that have similar characteristics are known, for example, from International patent application No. WO-A-97/46849. In such patent application, there is disclosed a head with the typical structure of the so-called xe2x80x9ccartridgexe2x80x9d heads, including a cylindrical spindle, axially sliding within a casing by means of a guide device consisting of two axial bearings, housed in the casing in longitudinally spaced out positions, each including through holes in which there is partially inserted the spindle. The spindle carries at one end a feeler for contacting the piece to be checked and at the other end a ferromagnetic core that translates inside associated windings as a consequence of axial displacements of the spindle. Each axial bearing includes closed tracks in which rows of appropriately shaped elements, as balls, can roll. The tracks include longitudinal portions formed at positions corresponding to the slits located on the internal surface of the bearing along which the balls contact the spindle and enable its axial sliding with respect to the casing. In said cartridge head, for the purpose of preventing axial rotations of the spindle with respect to the casing, there is a pin radially fixed to the spindle and carryingxe2x80x94at its free endxe2x80x94an idle wheel housed in an axial slit integral with the casing, for example achieved in a spacer element located between the two bearings.
In these linear gauges, the unavoidable clearance existing between slit and idle wheel may affect the accuracy and the repeatability, especially in those cases when there is the need to utilize a feeler offset with respect to the longitudinal axis of the gauge. In fact, in similar cases slight rotations of the spindle about its axis, due to clearance existing between slit and idle wheel, may cause considerable rotations of the feeler, and negatively affect the accuracy and repeatability of the checkings performed by the linear gauge.
Furthermore, an antirotation device as the one disclosed in WO-A-97/46849 necessitates suitable precision, mechanical machinings to be carried out on the spindle for the insertion of the pin with the idle wheel, and on the spacer element for achieving the slit, hence this implies additional expenses and considerable assembly time. Other known linear gauges can include other types of guide and sliding means for the movable spindle, for example bushings with cages with holes for seating the balls. Linear gauges of this type are disclosed, for example, in U.S. Pat. No. 4,347,492. In each of the two embodiments disclosed in the U.S. patent too, the axial rotations of the spindle are limited by a device, similar the one previously briefly described, that includes a pin radially coupled to the spindle and partially housedxe2x80x94with limited but unavoidable clearancexe2x80x94in an axial slit formed in the casing.
U.S. Pat. No. 5,779,367 discloses spline bearings assemblies with shafts featuring track grooves and relevant outer cylinders housing rolling elements that partially protrude and are arranged in the track grooves to guide movements of the shafts. The spline bearings include additional grooves housing linear scales with magnetic poles, and one or more magnetic sensors are fixed to each outer cylinder, e.g. housed in further grooves of the latter. U.S. Pat. No. 5,779,367 does not refer to a linear gauging head for dimensional checkings, but to complex and costly bearing assemblies, e.g. for small carriages, allowing to control the position of the carriages.
Object of the present invention is to provide a linear gauging head that guarantees high standards of accuracy, repeatability and reliability, enablesxe2x80x94with respect to the known structuresxe2x80x94a reduction of costs and time required for the assembly of the various component parts and offers a particularly simple structure.
This and other objects are achieved by a linear gauging head according to claim 1.
One of the main advantages provided by the herein illustrated and described linear gauging head consists in its specific compactness, achieved also thanks to the reduction of the number of component parts.
Another important advantage, offered by the substantial elimination of clearance in the limitation of the axial rotation of the spindle, consists in the possibility of utilizing simple and inexpensive linear gauging heads according to the invention in applications wherein the feeler is offset with respect to the spindle sliding axis and there are required a high degree of accuracy and repeatability.
A preferred embodiment of a linear gauging head according to the invention is now described with reference to the enclosed sheets of drawings, given by way of non limiting example, wherein:
FIG. 1 is a longitudinal cross-sectional view of a gauging head according to an embodiment of the invention;
FIG. 2 is an enlarged scale cross-sectional view of the gauging head shown in FIG. 1, taken along line IIxe2x80x94II in FIG. 1; and
FIGS. 3, 4, 5 are cross-sectional views of gauging heads according to three different embodiments of the invention.